Fix compiler warning in hv.c on MSWin32

Fixes the following seen in a Steve Hay smoke test:

Compiler messages(MSWin32):
..\hv.c(1646) : warning C4244: 'initializing' : conversion from 'unsigned long ' to 'const char ', possible loss of data

1 files changed, 1 insertions, 1 deletions

diff --git a/hv.c b/hv.c
index d0f452e6f9..d5e7a213f8 100644
--- a/hv.c
+++ b/hv.c
@@ -1643,7 +1643,7 @@ S_hfreeentries(pTHX_ HV *hv)
     /* This is the array that we're going to restore  */
     HE **const orig_array = HvARRAY(hv);
     HE **tmp_array = NULL;
-    const bool has_aux = SvOOK(hv);
+    const bool has_aux = (SvOOK(hv) == SVf_OOK);
     struct xpvhv_aux * current_aux = NULL;
     int attempts = 100;